Thermoplastic resins can be modified by a number of additives which act as flame retardants to hinder ignition or reduce flame spread. These flame retardants frequently must be employed in large quantities and processed at elevated temperatures, resulting in the degradation of the properties of thermoplastic polymers. Some flame retardants are difficult to disperse and to process into the resin, particularly the polymeric additives.
Chlorinated flame retardant additives such as chlorinated paraffins are frequently used, primarily because of their low cost. However, the chlorinated flame retardants generally have poor thermal stability. Chlorinated flame retardants with good thermal stability are frequently high cost additives. Thermoplastics using them often require the use of up to 15% by weight antimony oxide as a synergist.
Antimony oxide has severe limitations when used as a halogen synergist in thermoplastics. The use of high concentrations of antimony oxide in thermoplastics frequently leads to "after-glow" after flame extinguishment. The fine powder dusts readily during handling. It is recommended that breathing the dust be avoided. Antimony oxide has caused lung tumors in laboratory inhalation studies with animals. Further, in polyethylene terephthalate, polybutylene terephthalate and polycarbonate resins, antimony oxide acts as a depolymerization catalyst.
Generally, between 3 and 8% w/w antimony oxide is used as a halogen synergist in thermoplastics. In polypropylene up to 15% w/w antimony oxide is sometimes necessary for optimum flame retardancy. It has been found that oxide substantially degrades the physical properties of engineering thermoplastics, for example, the impact resistance. In addition, high loadings of the white antimony oxide make it impossible to make a product with deep rich colors.
Bromine compounds are known to be about twice as effective a flame retardant as chlorine compounds. Bromoparaffins are limited in their use to polyvinylchloride and polyurethane foam because of their poor heat stability. However, brominated aromatics have emerged to fill the needs of more specialized thermoplastic polymers. Examples are poly(tribromostyrene) which is marketed under the tradename Pyrochek 68 PB by Ferro Corporation, poly(dibromophenylene oxide), bis(tetrabromophthalimido)ethane, decabromodiphenyl oxide, bis(tribromophenoxy)ethane and brominated polycarbonate oligomers which are marketed under the tradenames BC-58 and BC-52 by Great Lakes Chemical. These flame retardants offer improved thermal stability and chemical resistance over the brominated paraffins. However, each of these compounds is limited in its cost effective application to special resin compositions as a result of their effect on physical properties, toxicity of brominated furans and processability. Also, in some applications over 5% w/w antimony oxide must be used to attain excellent flame retarding qualities.
Phosphorus compounds such as phosphate esters, with or without halogen are also used as flame retardant additives. Typical compounds are the alkyl, alkaryl and triaryl phosphates used as plasticizers in PVC, and the halogenated alkylphosphates and phosphonates used in urethane foams, phenolics, acrylics and epoxies. Low thermal stability, volatility and the tendency to plasticize limit the use of these additives. Phosphorus compounds are about three times more effective than bromine compounds.
U.S. Pat. Nos. 3,912,792 and 4,021,406 to Touval teach a three-part flame retarding composition for organic polymers consisting of from 0.5% to 20% of a tris(halophenyl)phosphate based on the combined weight of one to twelve parts by weight of a halogen-containing organic having a volatization temperature of more than 200.degree. C. and one part by weight antimony oxide. When the polymer is other than polypropylene, the flame retarding composition must also contain sulfur. The Examples and Table I in the patents teach that there must be at least 17 parts by weight antimony oxide per 5 parts by weight tris(halophenyl)phosphate to provide flame retardancy (together with 50 parts by weight halogen-containing organic).
Further, the patent teaches that tris(2,4,6-tribromophenyl)phosphate does not significantly reduce the flammability of polypropylene at the relatively low concentrations employed unless both antimony oxide and halogen-containing organic compound are present.